I had to format my computer and figured I could just install Rider. I got a notification when loading a project that I required the Microsoft .NET Framework Developer Pack 4.5 from a link within the application. (https://www.microsoft.com/net/targeting) The only option available there was 4.5.2, which I installed and when rebooting Rider I received the same issue.
I then found this post: https://rider-support.jetbrains.com/hc/en-us/articles/207288089-Using-Project-Rider-under-Windows-without-Visual-Studio-prerequisites which said I could install using the Build Tools. So I installed 4.5.1 according to the bullet point below. I received the same error in Rider.
It was only when I uninstalled these and installed plain 4.5 using the Build Tools that my errors went away in Rider.
The build tools also seem to install some semblance of Visual Studio?